In general, a resin vehicle part to be used as interior/exterior component (e.g., a bumper, a quarter trim, etc.) is provided with a panel body having a front surface including a design surface, the appearance of which is regarded as important in design, and a reinforcing rib provided standing on a back surface of the panel body. This resin vehicle part is manufactured by an injection molding method.
If the panel body is designed to be thinner in wall thickness in order to reduce the weight of the resin vehicle part, the front surface of a portion of the panel body, on which the reinforcing rib will be provided in an upright or standing manner, is apt to cause shrinkage strain associated with cooling of molten resin during injection molding. In particular, the surface of a portion of the panel body with a smaller sectional curvature is low in surface rigidity, so that large shrinkage strain (sink) occurs. Accordingly, there has been studied a technique related to thinning of a base end portion of a reinforcing rib in order to reduce sink in the front surface of the thinned panel body.
For instance, Patent Document 1 discloses a ribbed housing which is an injection-molded part including a cover member (corresponding to the panel body) having a front surface desired to provide appropriate appearance quality and a back surface formed with a rib, wherein the rib of a ribbed housing panel is continuously formed with a thin portion having a predetermined length and a thinner thickness in a direction away from the cover member and a thick portion having a larger thickness than the thin portion. The thickness of the thin portion is thinner in thickness than the cover member. Patent Document 1 describes that this ribbed housing panel in which the thickness of the thin portion is thinner in thickness than the cover member can prevent the generation of sink in the front surface of the cover member and the thick portion provided continuous with the thin portion can ensure the strength of the rib in supporting the cover member.
Furthermore, Patent Document 1 also discloses a configuration that a reinforcing rib is provided to extend from the cover member to the rib thick portion across the rib thin portion and teaches that the reinforcing rib can enhance the strength of the thin portion, thus increasing the strength of the rib itself.